The major goal of this project is to understand the biochemical events involved in the response of individual cells to external signals. We have previously shown that several types of hormone/neurotransmitter receptors increase in several cell lines when the cells are grown in the presence of certain short-chain fatty acids. This ability to modulate receptor expression has permitted us to analyze the biochemical events in adenylate cyclase activation by extracellular signals. In FY 82 we have concentrated on adapting several human cell lines to growth in serum-free media and examined the effects on receptor number and function. Our aim is to study the interactions of several hormones acting simultaneously on the same cell under well-defined conditions not possible with serum present. Elimination of serum causes a striking increase in beta-adrenbergic receptor number partly explainable by release from down-regulation due to catecholamines in serum; however, we have also found important roles for other media components in the regulation of receptor number and function.